1. Field of Invention
The present invention relates to suspension systems for gyroscopes and other precision inertially sensitive instruments. More particularly, the present invention relates to monolithically integral suspension systems having monolithically integral flexure hinges therein, and inertial instruments utilizing such suspension systems.
2. Prior Art
In the prior art dry tuned gyroscopes have been constructed and utilized in which the rotor of the gyroscope is attached to the spin shaft of the gyroscope through a two axis or a two degree of freedom suspension system which allows the rotor enough freedom of movement relative to the shaft to which it is attached so that the rotor in response to a disturbing rotation can maintain an independent orientation in space with respect to the shaft as the rotor rotates with the shaft. Restoring torques for deflections of the rotor relative to the shaft which are provided by centrifugal force are opposed by flexible elements in the suspension and are thus cancelled against each other (in the tuned condition) to leave the rotor substantially unconstrained for small deflectional rotation with respect to the shaft. The tuned frequency of the rotor is determined in part by the appropriate moments of inertia of the gimbal structure and by the elastic forces involved in the operation of the suspension. Conventionally such a gyroscope can be used in strapped-down (or flat form) application. In strapped-down applications electro-magnetic torquers are provided in the gyroscope to apply torquing forces which null the deflections of the rotor in response to applied rotations, the currents in the torquers then becoming a measure of the direction and magnitude of the rotations which are being sensed by the gyroscope.
The dry, tuned gyroscope because of its many advantages over the prior art floated instruments has rapidly begun to dominate the field of gyroscopic instruments. The theory of operation of elastically supported, dry tuned gyroscopes is well presented in the paper by the present inventor Craig entitled "Theory of Operation of an Elastically Supported, Tuned Gyroscope", I.E.E.E. Transactions on Aerospace and Electronic Systems (May, 1972) Vol. AES-8 No. 3, at page 280. In that paper's discussion of the elastically supported, tuned gyroscope, the two axis or two degree of freedom suspension system which connects the rotor of the gyroscope to the rotating shaft is shown as a gimbal system connected by one pair of torsion rods to the rotor (to define one axis of rotation of the rotor) and connected by another pair of torsion rods to the shaft (to define a second axis of rotation of the rotor relative to the shaft). A similar torsion spring gimballed gyro is shown schematically in FIG. 1 of Erdley et al., U.S. Pat. No. 3,463,016, and a related device is shown in U.S. Pat. No. 3,974,701, also to Erdley.
In the development of the inertial art two axis elastic suspension systems utilizing torsion rods as the flexible elements are being rapidly replaced by two axis elastic suspension systems which utilize flexure hinges or pivots as the flexible elements. Such flexure hinges are ordinarily constructed as a pair of crossed, metal straps which are connected beween two elements which are to be rotatable with respect to each other. Each strap acts as a bridge between the two elements which are to be rotatable with respect to each other and the geometry of the crossed straps is such that they establish a precise pivot axis at the line of intersection of the crossed straps or bridges. The accuracy and utility of an elastic suspension system utilizing flexure pivots is in the prior art determined by the accuracy of construction of the flexure pivots themselves and by the accuracy of assembly of the total suspension structure comprising gimbals and flexure pivots assembled together by such methods as cementing, bonding, etc. To the extent that such an elastic suspension system comprises many different parts which have to be assembled together there is a loss of accuracy due to accumulation of machining tolerances, errors in assembly and the loss of stability of the structure implicit in having cemented connections or other types of bonding of the separate parts. In the prior art several attempts have been made to improve the design and construction of such suspension systems but none has been totally effective. For example, U.S. Pat. No. 3,832,906 by the present inventor Craig shows an elastic suspension system constructed using a plurality of one-piece flexure pivots or hinges and a plurality of gimbal members all of which are cemented or bonded together in a particular configuration to create a two axis elastic suspension for the rotor of a tuned gyroscope. However, because of the large number of parts that have to be assembled, this design is subject to accumulation of machining tolerances and to instability in the bonding materials used which severely limits its ultimate utility and effectiveness.
U.S. Pat. No. 3,811,172 to Bilinski shows another method of attempting to solve this problem of stability and accuracy of an elastic suspension system in which Bilinski has his gimbals formed by two cylinders which are nested within one another and are bonded together. Before the cylinders are nested together and bonded they are each separately cut by a forming tool so that for each flexure hinge which is to ultimately exist in the suspension system, one strap or bridge of the flexure hinge is formed in one of the cylinders and the corresponding crossed-strap of the flexure hinge is formed in the other of the cylinders. When the two cylinders are positioned properly, nested within each other and bonded, the cooperation of the separated straps or hinges in the two separated cylinders is such that they simulate or emulate the performance of a conventional flexure hinge. This structure of Bilinski reduces the entire elastic suspension system to two elements which are bonded together. However, the performance of the system is critically dependent upon the accurate cutting of the two separate cylinders, upon their thoroughly accurate positioning relative to one another, and upon the long term stability of the cementing or bonding compound which is utilized. In practice, these requirements severely limit the ultimate performance of an elastic suspension system constructed in accordance with the teachings of Bilinski. Moreover, in the Bilinski device because the pivot axis is established by separate bridge elements in two separate cylindrical structures the flexure hinges formed are not true flexure hinges but only emulations thereof and have a certain lack of definition or region of uncertainty of the defined pivot axis which also limits the performance of such a device.
The Bilinski structure also has a subtle limitation as to its ability to withstand large axial accelerations or G-loadings, as might be encountered in many applications of a gyroscopic instrument. Because of the structure of the Bilinski device in response to an applied axial acceleration or force only half of the bridge elements of the flexures will bear the tensional or compressional forces, and thus only half of the bridges carry the stresses. This severely limits the load bearing utility of the suspension system in many gyroscopic applications.
There also exists another structure similar to Bilinski which, however, has its multi-part flexures oriented so as to have a very symmetrical response to G-loadings or accelerations in either direction along the spin axis. Because the response of this suspension system is symmetrical with respect to G-loadings, the frequency response of this gimbal system will be the same independent of the orientation with respect to gravity of a gyroscope employing such a suspension system. Thus, with this structure, when two gyroscopes are used in a strapped-down or platform inertial navigation system in which the gyroscopes are at different orientations with respect to gravity, they will nevertheless both have the same tuned frequency. However, because of cross coupling effects between the two gyroscopes in practice, it is customary to actually operate both of the gyroscopes detuned in opposite direction either by physically detuning the suspensions or by driving them with two different spin frequencies which slightly detunes both gyros. This reduces cross coupling between the gyroscopes. However, as a result either the gyroscopes are no longer uniform causing great problems in maintenance of stock gyroscopes, replacement of gyroscopes, etc., or the gyroscopes are operated detuned reducing their performance and sensitivity.
The ultimate performance of a gyroscope is also intimately connected with the nature of the mechanical, thermal, and gaseous environment in which it operates, as established by the construction of the support structures and housing of the gyroscope. Ordinarily in the prior art the main support structure for a gyroscope is established by a web in a central cylindrical section of a cylindrical housing, with the gyro motor usually below this web and the gyro rotor usually above the central web, the housing being completed by positioning top and bottom housing caps which are sealed and bonded against the central cylindrical housing member to provide a vacuum-tight enclosure. The use of two seals multiplies leakage problems in attempting to maintain the reduced pressure, hydrogen atmosphere which is commonly used in the interior of dry tuned gyroscopes. In turn, windage, thermal gradient and environmental stability of materials over long periods of time are critically affected. Moreover, the structures of the described type tend to fairly well act like a volume having two separated top and bottom compartments with much increased opportunity for varying thermal and dimensional effects taking place in the separated compartments so as to limit the stability and uniformity of the total structure. With respect to prior art, complete gyro structures, representative examples of such structures can be seen in U.S. Pat. No. 3,354,726 to Krupick, et al. and U.S. Pat. Nos. 3,463,016 and 3,676,764, both to Erdley et al. It should be noted that in such a system any tests of spin bearings or of other elements of the spin assembly can only be conducted after the spin assembly is loaded into the overall gyro structure (because the spin assembly is not completed until that time). This creates severe operational problems in routine testing and maintenance of gyroscopes.